The present invention is directed to a catalytic asymmetric epoxidation of olefins by contacting an oxidizing agent to a mixture of a cyclic chiral ketone and an olefin.
Epoxides are very important chiral building blocks for the synthesis of enantiomerically pure complex molecules. Asymmetric epoxidation of olefins presents a powerful strategy for the synthesis of enantiomerically enriched epoxides. Great success has been achieved in the epoxidation of allylic alcohols, unfunctionalized cis-olefins, and conjugated trisubstituted olefins. However, the epoxidation of trans-olefins bearing no allylic alcohol group with high enantiomeric excess still remains a challenging problem.
Among many other powerful epoxidation methods chiral dioxiranes generated in situ from an oxidizing agent, typically Oxone(copyright) (potassium peroxomonosulfate), and a chiral ketone have appeared to be promising reagents for asymmetric epoxidations, particularly for trans-olefins bearing no allylic alcohol groups. Since the first asymmetric epoxidation of olefins with dioxirane were reported in 1984, significant progress has been made in the area. A C2 symmetric cyclic chiral ketone derived from 1,1xe2x80x2-binaphthyl-2,2xe2x80x2-dicarboxylic acid has been used as a catalyst to achieve high enantioselectivity for the epoxidation of trans-4,4xe2x80x2-disubstituted stilbenes. This cyclic chiral ketone, however, is limited to certain substrates.
Therefore, there is a need for an inexpensive, readily available, and general asymmetric epoxidation catalyst which can epoxidize a variety of olefins with high enantioselectivity.
In general, the present invention is based on enantioselective epoxidation of an olefin by an epoxidizing agent derived from a reaction between an oxidizing agent and a cyclic chiral ketone. Without being bound by any theory, it is believed that a reaction beween the oxidizing agent and the chiral ketone produces a dioxirane that is believed to be the intermediate responsible for the observed enantioselective epoxidation reaction. Thus, the present invention can also be described as contacting the olefin with an epoxidizing agent which is derived from contacting the oxidizing agent with the chiral ketone. However, to reflect a preferred procedure of adding the oxidizing agent to the reaction mixture, the present invention is described herein as contacting the oxidizing agent with an olefin in the presence of the chiral ketone.
In one embodiment of the present invention, the oxidizing agent is derived from a mixture of hydrogen peroxide and a nitrile compound.
The reaction mixture can also include a base. The pH of the reaction mixture is preferably from about pH 5 to about pH 14, more preferably at pH of from about pH 10 to about pH 14, and most preferably from about pH 10 to about pH 12.
In one particular aspect, the present invention provides a method of producing an epoxide from an olefin comprising contacting an oxidizing agent with an olefin in the presence of a chiral ketone under conditions sufficient to produce an epoxide from the olefin.
In another aspect, the present invention provides a method for increasing a relative concentration of at least one stereoisomer of an olefin from the stereoisomer mixture. The method generally involves converting one of the stereoisomers of the olefin to an epoxide at a higher rate than the conversion of the other stereoisomer. The method generally involves contacting an oxidizing agent with a stereoisomer mixture of the olefin in the presence of a chiral ketone described herein. The reaction epoxidizes one of the olefin stereoisomers at a substantially higher rate than the other stereoisomer. This difference in reactivity of stereoisomers results in a relative increase in the concentration of the other olefin stereoisomer.
Yet in another aspect, the present invention provides a method for stereoselectively producing an xcex1-acyloxy carbonyl compound from an enol ester olefin. The method generally involves:
(a) contacting an oxidizing agent with the enol ester olefin in the presence of the chiral ketone described herein under conditions sufficient to stereoselectively produce an enol ester epoxide, and
(b) contacting the enol ester epoxide with an acid catalyst under conditions sufficient to stereoselectively produce the xcex1-acyloxy carbonyl compound.
The xcex1-acyloxy carbonyl compound can be produced from the enol ester epoxide with inversion or retention of stereochemistry depending on the acid catalyst used.